Plastic surfaces are frequently coated with suitable metal layers either for technical reasons or for decorative reasons. In the case of technically reasoned coatings the same can be applied for instance for forming electrically conducting structures on the plastic surfaces. If the coating is applied for decorative reasons, plastic surfaces having a high quality appearance will be produced in this way. Especially in the field of automotive construction decoratively metalized plastic surfaces are widely spread. In the domain of electronic and electrical engineering conductive structures are deposited for instance on plastic boards for forming integrated circuits by means of suitable metal deposition processes.
For the durability of the metal layers deposited on the plastic surfaces it is decisive that the same exhibit sufficient adhesion on the plastic surfaces. To provide for a corresponding adhering strength, it is the common practice in prior art to roughen the plastic surfaces prior to a corresponding metal deposition, in order to be able to provide for a sufficient adhering strength of the deposited metal layers. For this purpose different processes are known from prior art. Typically, plastic surfaces are treated with chromium(VI) containing pickling solutions. Such solutions which are based on chromium-sulfuric acid can include for instance chromium(VI) oxide and sulfuric acid at a weight ratio of 1:1.
At the immersion of the plastic surface into such chromium acid pickling solution the polybutadiene component on the plastic surface will oxidatively decompose and leave cavities on the surface, which serve as corresponding anchoring structures for the deposited metal layers. It is assumed that the adhesion is produced by this anchoring effect.
On the other hand, chromium(VI) containing compounds are suspected of being cancer-causing, so that dealing with these compounds is subject to strict environmental regulations. In the face of the potential danger caused by chromium(VI) releasing compounds, a prohibition of the industrial use of chromium(VI) containing compounds cannot be excluded.
An alternative for the mentioned chromium(VI) containing pickling solutions for plastic surfaces are pickling solutions based on alkali permanganates. For this purpose, pickling solutions based on alkali permanganates, preferably potassium permanganate, and a mineral acid like for instance phosphoric acid are used.
The pickling solutions thus prepared are free of chromium(VI) containing compounds and exhibit a good pickling effect for plastic surfaces, especially plastic surfaces from ABS plastic.
Alkali permanganate-based pickling solutions typically contain up to 20 g/l of alkali permanganate in a corresponding mineral acid solution such as a 40 to 85% phosphoric acid.
However, the alkali permanganate which is used is subject to a rapid self-decomposition, so that the pickling effect of a freshly prepared alkali permanganate pickling solution will be insufficient already after several hours. As a decomposition product of the alkali permanganates colloidal manganese(IV) compounds are apparently produced which neither allow filtering nor centrifuging. The pickling solution which has lost its pickling effect can be re-sharpened by the addition of alkali permanganate. During this however, the colloidal manganese(IV) decomposition products, manganese(II) ions as well as the alkali ions become even more enriched within the pickling solution. As a result of this enrichment the viscosity of the pickling solution will be increased up to a point where the pickling solution must be discarded, because satisfying pickling results cannot be achieved any longer.
A further drawback of the described alkali permanganate pickling solutions is the colloidal appearance of the decomposition products. Since these decomposition products, as described above, can neither be filtered nor centrifuged, these colloidal manganese(IV) compounds are frequently spread over the entire coating process of the plastic surfaces.
But this spreading may lead to a strong influence on the deposition process, so that metal layers are finally deposited which are faulty or otherwise insufficient.